The ability to precisely manipulate fluidic streams enhances the use and effectiveness of microfluidic devices. Typically, networks of small channels provide a flexible platform for manipulation of small amounts of fluids. Certain microfluidic devices utilize aqueous droplets in an immiscible carrier fluid. The droplets provide a well-defined, encapsulated microenvironment that eliminates cross contamination and changes in concentration due to diffusion or surface interactions.
Microfluidic devices for performing biological, chemical, and diagnostic assays generally include at least one substrate containing one or more etched or molded channels. The channels are generally arranged to form individual fluid circuits, each circuit including a sample fluid channel, an immiscible carrier fluid channel, and an outlet channel. The channels of each circuit may be configured such that they meet at a junction so that droplets of aqueous fluid surrounded by carrier fluid are formed at the junction and flow into the outlet channel. In some cases, the outlet channel of each circuit is connected to a main channel that receives all of the droplets from the different fluidic circuits and flows them to an analysis module. In other cases, the outlet channels connect to exit ports to carry the droplets to a collection vessel.
Since each fluidic circuit may have different samples, and because different compositions (e.g., concentration and/or length of nucleic acid) from different samples affect how droplets form, droplets of different sizes may be produced by each circuit. A problem with droplets of different sizes flowing through the same channel is that the droplets travel at different velocities. Droplets traveling at different velocities may cause unwanted collisions or unwanted coalescence of droplets in the channel. Thus, it is important that individual fluidic circuits produce droplets of uniform size so that the droplets travel at the same velocity in the channel and do not collide or coalesce in an unwanted manner.
Droplets are typically generated one at a time at a junction between an aqueous fluid and an immiscible carrier fluid. Droplet volume and frequency (the number of droplet generated per unit time) are determined by geometrical factors such as the cross-sectional area of the channels at the junction and the fluidic properties such as the fluid viscosities and surface tensions as well as the infusion rates of the aqueous and carrier fluids. To control the volume of the aqueous droplet, within a range, droplet volume can be adjusted by tuning the oil infusion rate through the junction. This is readily achieved with a pressure regulator on the carrier fluid stream. In some cases it is desirable to have multiple junctions operating as separate circuits to generate droplets and have independent control over the oil infusion rates through each circuit. This is readily achieved by using separate pressure regulators for each aqueous stream and each carrier fluid stream. A simpler and lower cost system would have a single carrier oil source at a single pressure providing a flow of carrier oil through each system. The problem with such a system is that in adjusting the pressure to regulate the flow of carrier oil in one circuit the carrier oil in all circuits would be effected and independent control over droplet volume would be compromised. Thus, it is important to have a means whereby at a fixed carrier oil pressure the flow of carrier oil in each of the circuits can be independently controlled to regulate droplet volume.